The present invention relates to a high-performance aspherical zoom lens for use in a 3-CCD (charge coupled device) type video camera or the like, which has a high zoom ratio of about 10 and a long back focal distance.
In response to recent demand for excellent operational efficiency, good mobility and high image quality in video cameras, high-resolution imaging devices as compact as 1/3" are becoming a mainstream of imaging devices. Meanwhile, in this connection, high-performance and high-magnification zoom lenses which are large in aperture ratio, compact in size and light in weight are in strong demand. Furthermore, due to a big demand for reduction of production cost of zoom lenses, high-performance and high-magnification zoom lenses in which the number of lens components is reduced are in urgent need.
However, in known high-magnification zoom lenses, not only diameters and an overall length of lenses become large but a large number of the lenses are required to be used for performing more strict aberration correction. As a result, the known high-magnification zoom lenses become larger, heavier and more expensive and therefore, have not been suitable for use in video cameras for home use. Therefore, known compact and light zoom lenses having an F-number of about 1.4 to 1.6 and including 10 lenses or so have a zoom ratio of about 6.
Hereinbelow, one example of a prior art zoom lens for use in a video camera disclosed in, for example, U.S. Pat. No. 5,100,223 is described with reference to FIG. 2 . The prior art zoom lens includes a first lens group 21 acting as an image forming portion, a second lens group 22 acting as a magnification changing portion, a third lens group 23 acting as a light converging portion, a fourth lens group 24 acting a focusing portion and a glass plate 25 optically equivalent to a quartz crystal filter and a face plate of an imaging device. Reference numeral 26 denotes an image surface.
Operation of the prior art zoom lens of the above described arrangement is described, hereinbelow. The first lens group 21 is fixed relative to the image surface 26 and has an image forming function, while the second lens group 22 is movable on an optical axis A so as to change magnification such that a focal length of an entire system is changed. The third lens group 23 is fixed relative to the image surface 26 and has a function of converging divergent light produced by the second lens group 22. On the other hand, the fourth lens group 24 is movable on the optical axis A and has a focusing function Variations in position of the image surface 26 due to travel of the second lens group 22 at the time of zooming are eliminated by displacing the fourth lens group 24 such that the image surface 26 is fixed at a predetermined position.
However, in the prior art zoom lens of the above described arrangement, if the zoom ratio is set to 10 or so, it becomes difficult to perform aberration correction over an entire zoom range and an entire shooting distance, so that high image quality cannot be achieved. Furthermore, the prior art zoom lens has such a drawback that since the back focal distance is short, a color separation optical system required for producing a three-CCD type video camera cannot be inserted between the lens system and the image surface 26.